Understanding the Endocrine System (mod 7)

Oct 15, 2024

Lecture Notes: Chapter 17 - The Endocrine System

Overview

  • Transition from the nervous system to the endocrine system.
  • Focus on the integration between the nervous system and the endocrine system.

Endocrine System

  • Composed of glands or glandular structures.
  • Secretes hormones into the bloodstream (endocrine glands) as opposed to exocrine glands, which secrete externally.
  • Hormones are chemical messengers.

Communication in the Body

  • Nervous System: Uses electrical signals via neurons for fast communication.
  • Endocrine System: Uses hormones for communication, influencing distant tissues through the bloodstream.

Modes of Chemical Signaling

  1. Direct Communication: Cells directly adhere and share cytoplasm through gap junctions.
  2. Paracrine Communication: Local communication through extracellular fluid; involves paracrine factors.
  3. Endocrine Communication: Hormones travel through the bloodstream to distant target cells.
  4. Synaptic Communication: Neurotransmitters cross synapses between neurons.
  5. Autocrine Communication: Cells respond to their own secretions.

Nervous vs. Endocrine Systems

  • Speed: Nervous is fast; endocrine is slow due to hormone circulation.
  • Duration: Nervous has short impacts; endocrine has long-lasting effects.
  • Modulation: Nervous uses frequency modulation; endocrine uses amplitude modulation.
  • Impacts: Nervous impacts are specific; endocrine impacts are widespread.

Hormone Structural Categories

  1. Amino Acid Derivatives: Modified amino acids (e.g., tyrosine, tryptophan).
    • Examples: Thyroxine, catecholamines, melatonin.
  2. Peptide Hormones: Chains of amino acids (e.g., glycoproteins, small proteins).
    • Examples: Insulin, growth hormone.
  3. Lipid Derivatives: Derived from lipids (e.g., eicosanoids, steroids).
    • Examples: Testosterone, estrogen, prostaglandins.

Hormone Solubility

  • Polar (water-soluble) hormones bind to membrane receptors.
  • Nonpolar (lipid-soluble) hormones can cross membranes directly.

Hormone Circulation

  • Freely Circulating: Rare, quick removal; half-life ~30 minutes.
  • Bound to Plasma Proteins: Common, longer half-life (days/weeks).

Patterns of Hormone Secretion

  1. Chronic Secretion: Constant rate of release (e.g., thyroid hormones).
  2. Acute Secretion: Released in response to stimuli (e.g., insulin).
  3. Episodic Secretion: Occurs at specific intervals (e.g., menstrual hormones).

Control of Hormone Secretion

  • Humoral: Response to local conditions (paracrine factors).
  • Neural: Direct stimulation by the nervous system.
  • Hormonal: Controlled by other hormones (hormone pathways).

Hormone Pathways

  • Releasing Hormones: From hypothalamus, trigger tropic hormones.
  • Tropic Hormones: From anterior pituitary, stimulate other glands.
  • Terminal Hormones: Produced by target endocrine glands.

G Protein and Second Messengers

  • Membrane-Bound Receptors: Use G proteins to activate intracellular pathways.
  • Second Messengers: Cyclic AMP or calcium/calmodulin complexes.
  • Intracellular Receptor Binding: Nonpolar hormones bind to internal receptors affecting gene expression.

Hormone Action on Cells

  • Steroid Hormones: Directly enter cells and impact gene expression.
  • Thyroid Hormones: Transported into cells, alter metabolism and gene expression.

Summary

  • The endocrine system provides long-lasting and widespread modulation of body processes through hormones.
  • Understanding hormone pathways and the interplay between different systems is critical for comprehending body regulation.